1. Field of the Invention
The preferred embodiments of the present invention are directed to obtaining seismic information regarding hydrocarbon producing reservoirs. More particularly, the preferred embodiments are directed to placing sensor equipment in shallow, low angle boreholes for acquisition of seismic information.
2. Background of the Invention
The key to efficient and economical extraction of hydrocarbons from subsurface formations is information. The primary informational sources relied on in the oil and gas industry are seismic graphs of the formation at issue. Early seismic graphs (sometimes referred to as seismographs) were two-dimensional in that the information they conveyed could be represented in only two axes. With the advent and application of computer processing in the seismic arts, geophysicists were able to further refine seismic techniques to obtain three-dimensional (3-D) seismic graphs of formations at issue. Such 3-D seismic graphs aid the oil and gas industry in the location and placement of both vertical and horizontal hydrocarbon-producing wells. That is, the 3-D seismic graphs may indicate the highest point of the producing formation, the highest point being the most efficient extraction point. As an additional example, some hydrocarbon-producing layers may be substantially horizontal, narrow bands. In such circumstances, a three-dimensional seismograph allows geologists to optimally place the horizontal components of a well bore.
Once hydrocarbon producing wells are in place, the need for information does not cease. The oil and gas industry tracks the extraction of the hydrocarbons of the formation over time for many reasons, e.g. to estimate remaining reserves, to track water encroachment, and the like. Tracking formations using seismic technology over time is generally referred to as four-dimensional (4-D) seismic. The fourth dimension in 4-D seismic is, of course, the time dimension, and it is this time dimension which introduces difficulties in obtaining the information. Several methods exist for obtaining 4-D seismic information. One such technique involves obtaining multiple 3-D seismic surveys using standard surface techniques. As one of ordinary skill in the art is aware, obtaining a seismic survey using surface techniques involves placing long strands of cable across the surface of the earth, the cable having periodically spaced seismic receiving devices. Generally speaking, the seismic receiving devices are placed in a grid pattern over or proximate to the formation of interest. FIG. 1 exemplifies placement of a cable 10 having seismic sensors 12 on the surface 14 of the earth. After the seismic receiving devices are placed, a seismic event is triggered, for example by detonation of dynamite or through the use of vibrator trucks which contact the surface of the earth and impart energy. The energy, whether created by dynamite or by trucks, propagates through the various earth layers to the formation of interest, and portions of the signal reflect back to the surface receivers.
Obtaining acoustic information in this manner is very time consuming and requires extensive access to the surface. For 4-D seismic using surface techniques to be viable, the process must be performed periodically, for example on a yearly basis. However, the repeatability of data obtained in this manner is questionable. First, the earth's surface, even over the course of a year, may change due to seasonal and other “weathering” factors, such as changes in water table levels. Further, it is impractical to guarantee that seismic sensors on the cables will have the precise coupling from year to year, or even that seismic sensors will be in the same precise location. Moreover, access to the surface to perform these type surveys may be limited, especially in environmentally sensitive areas or at certain times of the year. Thus, while obtaining 4-D seismic data using surface techniques is possible, it has many difficulties.
Related art techniques exist to permanently install the seismic sensors at or near the surface. The permanent installation could be by way of burying the cables and related seismic sensing devices in a trench a few inches or feet below the ground, or by placing the sensors in periodically spaced, relatively shallow vertical wells. These methods, however, require extensive surface access and may significantly damage or impair the surface in environmentally sensitive areas. In the marine context, these sensors are typically placed by hydrojet a few inches or feet below the surface. Anchors for ships, shrimping operations, and the like damage sensors placed this way.
A second method for obtaining seismic information is to place seismic sensors within the well bores of hydrocarbon producing wells. With the advent of deep horizontal drilling, it is common to drill a single vertical well bore from the surface to several thousand feet deep, and then extend from the vertical well a series of horizontal wells into the hydrocarbon producing formation to maximize hydrocarbon extraction. The horizontal wells, however, do not make right angles to the vertical well bore, but instead have a radius of curvature such that, if they go as planned, they become substantially horizontal as they enter the formation of interest. Thus, there are locations in the curved portion of the well bores (see 16 of FIG. 1) where the prior art seismic sensing devices are placed. However, placement of seismic sensing devices in these portions of the well bores has difficulties. The first difficulty is that these well bores are usually hydrocarbon producing well bores, and thus the seismic sensors may interfere with other downhole devices, such as production tubing, valves, scaler sensors (such as pressure and temperature), and the like. Secondly, the pattern of the horizontal well bores is optimized for production, and not seismic sensing.
A third method in the oil and gas industry for placement of seismic sensors is drilling a dedicated survey well, such as the system suggested in U.S. Pat. No. 6,065,538. While the survey well suggested in the '538 patent may address the problems associated with attempting to permanently install seismic sensors in a production well bore, and also the repeatability problems of a surface seismic survey, the cost of drilling a deep survey well (on the order of 8,000 feet), with no offset in hydrocarbon production, is in many cases prohibitive.
Thus, what is needed in the art is a mechanism whereby seismic sensors can be permanently or semi-permanently installed for 3-D and 4-D seismic purposes without the surface-intensive practices of surface seismic surveys or the relatively high cost of drilling deep survey wells.